Transmitters and receivers in many communication systems employ gain control techniques to adjust the signal levels of various amplifier stages. Analog and digital automatic gain control (AGC) circuits are well-known. For example, AGC circuits are frequently used in transmitters or receivers in the field of cellular, wireless, and spread-spectrum wireless communication systems.
The AGC function may be employed, for example, to bring a newly acquired signal at the receiver input into the dynamic range of the receiver, when a communication link is first established. Thereafter, the AGC function typically continually adjusts the overall receiver gain to compensate for fluctuations in received signal strength associated with fading, interference, or similar interruptions of the signal level.
Such gain control circuitry is often employed to maintain a constant level in the front end of a receiver, including the amplification and RF/IF demodulation stages. The constant level is maintained such that the output (demodulated) baseband signal level of the receiver remains within well-defined limits, even though many factors may vary gain levels within each component of the front end.
Typically, gain control problems were solved using analog receive signal strength detectors, rectification and averaging techniques, or other control loops based on signal amplitude or energy averaging. U.S. Pat. No. 7,738,605, entitled “Method and Apparatus for Adjusting Receiver Gain Based on Received Signal Envelope Detection,” discloses techniques for adjusting receiver gain based on received signal envelope detection. The received signal is sampled, for example, using a plurality of latches, to determine the signal envelope amplitude. The receiver gain is then adjusted based on the determined signal amplitude.
Such existing envelope detection techniques have significantly improved the ability to maintain a desired level in the front end of a receiver. The disclosed techniques, however, suffer from a number of limitations, which if overcome, could further improve the performance of the AGC function in receivers. In particular, in some channel environments, envelope-based AGC processes may not provide optimal gain control when the receiver input signal is over-equalized. Generally, in the presence of an over-equalized signal, the resulting signal gain at the sampling point is reduced, thereby causing a loss of signal detection margin.
A need therefore exists for improved methods and apparatus for automatic gain control. A further need exists for techniques for adjusting the receiver gain based on samples taken at the desired sampling phase and target voltage.